The present invention relates to a power supply unit connected to a commercial frequency power supply for an electric and electronic apparatus such as a personal computer, a television, a VTR, etc. Particularly, the invention relates to an energy-saving type power supply unit capable of substantially reducing power consumption during a light-load period such as a standby period and also capable of having high efficiency during both a light-load operation and a rated-load operation.
A conventional power supply unit has been designed to have a maximum efficiency during a rated-load period. Therefore, the conventional power supply unit has a drawback in that the efficiency is lowered substantially during a light-load period such as a standby period. On the other hand, when the power supply unit is designed to have maximum efficiency during a light-load period, the output voltage is lowered and the efficiency is lowered substantially during a rated-load period. Thus, in this case, the conventional power supply unit also has had a drawback.
In the light of the above problems, there have been various trials to overcome such difficulties. As a first example, Jpn. Pat. Appln. KOKOKU Publication No. 5-82146 disclosed "A DC-DC converter" on Nov. 17, 1993. This application is for substantially improving the conversion efficiency during a light-load period by intermittently operating a switching converter during the light-load operation.
As a second example, Matsushita Electronic Industries Co., Ltd. disclosed an article on reserved power supply IPD series in its news release in November 1998. The article explains that it is possible to reduce switching loss by decreasing the number of switching during a light-load period. This conventional example improves the efficiency during the light-load operation by decreasing the number of switching or the number of oscillation during this period.
As a third example, there is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-103541 a case for improving the efficiency during a light-load period by intermittently operating a transformer of a commercial frequency (50/60 Hz), as shown in FIG. 6.
As a fourth example, there is a publication of "A magic switch thyristor: application to 3.2 amplifier power supply" by Isao Shibasaki, Seibundo-Shinkousha, p. 139, issued on Dec. 24, 1985, ninth publication on Apr. 10, 1993. This discloses a technique for increasing output regulation and power efficiency by controlling the current conduction angle of TIAC that is inserted into the primary side of a power supply transformer of a power amplifier.
In the above first and second examples of known techniques, the efficiency during a light-load period is improved by employing an intermittent operation in the high-frequency switching regulator. In the third example, the efficiency is improved by intermittently operating the commercial frequency transformer during a light-load operation. In the fourth example, both regulation and efficiency are improved by controlling the conductive angle of a TRIAC for each cycle of commercial frequency.
However, according to the first and second examples, the handling of a high frequency involves unnecessary radiation of electromagnetic waves. According to the third example, the magnitude of unnecessary radiation of electromagnetic waves is small as the device handles a low frequency or a commercial frequency. However, as the power control is not steady because of a selective control of either an intermittent control or a continuous control, the regulation of an output voltage is not satisfactory. Therefore, there is such a problem that the output voltage increases during a light-load operation and the output voltage decreases during a rated-load operation. This makes it necessary to add a voltage stabilization device such as a three-terminal regulator to the secondary side of the transformer, which results in a complex construction.
According to the fourth example, an intermittent operation is not carried out as the conductive angle of each cycle is controlled. Therefore, there is no improvement in the efficiency during a super-light load operation like a standby period.
It is, therefore, an object of the present invention to provide an energy-saving type power supply unit capable of substantially reducing power consumption during a light-load period like a standby period and also having always high efficiency during both light-load and rated-load periods, in the power supply unit connected to a commercial frequency power supply for an electric/electronic apparatus such as a television, a VTR, etc.